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Old 30th October 2011, 10:36 PM   #1091
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Ken,
It would be difficult to implement an active load for the fvront end with the BH feedback scheme.
One way of looking at this scheme is to consider that the anode loads are in fact "anti-bootstrapped".
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Ian
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Old 31st October 2011, 05:35 AM   #1092
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Ian,
Thanks for the explanation. Now it make a lot of sense.

Hylle, you question is too general. Anything can drive anything, the question is what do you wish to achieve.
Since you mentioned your newbee status, I would suggest you build the circuit as it is & ask questions after the practical exercise. you may or may not like BH sound... & who know, you may actually convert to SS in the signal path.
Our local proverb says "If you have never tried, you will never know."
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Old 31st October 2011, 08:14 AM   #1093
Hylle is offline Hylle  Denmark
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Audio_Idiot, you are right of cause. I will start to implement the circuit, with the mosfets when I have gathered the parts.
- I dont feel my question is too general, I guess the question insted could sound like, what will happen in this circuit if I leave out the mosfets?, and why is that happening?
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Old 31st October 2011, 03:15 PM   #1094
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Being UL connected, the output stage will still has sizable miller's capacitance at the grid, paralleing multiple output tubes will multiply the miller's capacitace. The ECC83 has such puny current it may not get over the combine capacitance at the grid. This combine capacitance with the ra of the output tubes may lead to a high frequency roll off at audioble range. but you can still make sound with this, just not a lot of high frequency response.
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Old 31st October 2011, 03:29 PM   #1095
rongon is offline rongon  United States
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Would something like 12AT7 or ECC85 (6AQ8) help? Or do those tubes not have enough voltage gain (mu)?

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Old 1st November 2011, 01:57 AM   #1096
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12AT7,Ecc82,ECC803 they are about the same thing when it comes to specifications. I've no idea about ECC85 or 6AQ8, since they are not readily available to me, hence not interested.
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Old 1st November 2011, 06:32 AM   #1097
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A few general comments about the Source follower and "stuff" above.
1st - the BH scheme relies on trading output tube gm for reduced rp and hence reduced output impedance reflected to the secondary of the output tranny - that is why high gm tubes like the EL84 work better than 6V6, but I've buit several lovely 6V6 versions.
2) One way to get more gm is to parallel output tubes as Hylle is proposing. (The parallel output tubes can be considerd to be a single tube with double the gm).
3) You do need plenty of gain in the front end diff amp. With "normal" levels of shunt feedback, you will find that the amps sensitivity suffers if you use tubes with a mu of less than say 70, 6SL7, 12AT7 are OK but I would not want to use anything with lower mu. This is one reason why a cascode front end would probably work well, it give you a bit more gain to play with.
4)The source follower buffer offers a number of advantages:
- it allows use of fixed bias for the output tubes, without the buffer you need to keep Rg1 on the output tube grids high so as to not load the front end too much OR divide the shunnt feedback down too much. Without the buffer those Rg1 need to be 470K and that means cathode bias and either EL84 or EL34.
- If you current source load the source follower you have VERY high impedance which isolates any noise on the negative rail and it gives extremely tight control of the output tube grids and this helpd reduce noise contributed by the output tube grids. The 6V6/6SL7 version I'm running at the moment (with current source loaded source followers) has the blackest background (lowest noise) of any tube amp I've ever heard (that is any of about 20 DIY Amps and a similar number of commercail tube amps).
The bias is then applied to the source follower gate which is capacitively coupled back to the diff amp. The very high input impedance of the source follower allows the use of smaller coupling caps and so it is easier (and cheaper) to obtain better quality caps

Random afternoon ravings - hope there is something useful to BH builders.

Cheers,
Ian
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Old 1st November 2011, 01:18 PM   #1098
Hylle is offline Hylle  Denmark
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Thank you for explaining, Audio_Idiot and Ian. I am learning more and more.
Ian, comments for your point 4) Since we need as high an RG1 as possible, why can a grid choke not be used instead of RG1? It has thousands of H and a low DC resistance of maybe 2 Kohm, and at the same time has Mohms load AC wise? If this choke is put in the fixed bias voltage lines to each grid, then even fixed bias should be usable on the first version without mosfets?
Am I totally of hear, Do you follow what i suggest? I just would like you oppinion Ian, I will not bother you more about this Grid choke idea if you "shoot it down" now !!
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Old 1st November 2011, 01:25 PM   #1099
20to20 is offline 20to20  United States
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This Baby Huey thread is just an advertisement thread for the amp now. It needs its own sticky forum. The posts are too fragmented topic wise.
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Old 1st November 2011, 10:34 PM   #1100
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20to20 - Fair comment, all I can do is answer posters queries to the best of my knowledge/ability as they turn up.

Hylle,
I have no experience with grid chokes. The need to maintain high AC impedance down to the bottom of the audio range wories me. Those chokes would need to be 3 TO 4 THOUSAND Henries. I've tried to keep the basic design cheap so haven't investigated grid chokes.

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Ian
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